To find the time it takes for the radiator temperature to rise from -15°F to 40°F, we first calculate the total temperature increase needed:
\[ 40°F - (-15°F) = 40°F + 15°F = 55°F \]
Next, we know that the temperature rises at a rate of 2.5°F per minute. To find the time required to achieve a 55°F increase, we use the formula:
\[ \text{Time} = \frac{\text{Temperature Increase}}{\text{Rate of Increase}} \]
Substituting the values, we have:
\[ \text{Time} = \frac{55°F}{2.5°F/\text{minute}} = 22 \text{ minutes} \]
Therefore, it will take 22 minutes for the radiator temperature to reach 40°F.